The Pahlavis modernized Iran, improving infrastructure, education, and industry, while strengthening the military and legal systems.

[6] About a month later, under British direction, Reza Khan's 3,000–4,000 strong detachment of the Cossack Brigade reached Tehran in what became known as the 1921 Persian coup d'état.

[7][8] The rest of the country was taken by 1923, and by October 1925 the Majlis agreed to depose and formally exile Ahmad Shah Qajar.

The Majlis declared Reza Pahlavi as the new Shah of Iran on 12 December 1925, pursuant to the Persian Constitution of 1906.

[9] Initially, Pahlavi had planned to declare the country a republic, as his contemporary Atatürk had done in Turkey, but abandoned the idea in the face of British and clerical opposition.